again there are problems with python dependent packages. This time dev-lang/python-exec and dev-python/python-exec are causing trouble. When I try to update the former package, I get the following error:

that will offer to rebuild packages whose ebuilds in installed db differ from portage ebuilds. those ebuilds often introduce obsolete dependency restrictions (yes, they sometimes change without version bump!)

you could also try --explain "dev-lang/python-exec" --explain "dev-python/python-exec" which might add extra info on why specific version was taken into consideration or eliminated and what is blocking it.

Quote:

Flag 'python_targets_pypy2_0' enabled if it is enabled for 'dev-python/python-exec-10000.1:0::gentoo', assuming disabled if missing; Flag 'python_targets_python3_3' enabled if it is enabled for 'dev-python/python-exec-10000.1:0::gentoo', assuming disabled if missing

! dev-lang/python-exec
Reasons: dev-python/python-exec-10000.1:0::gentoo, dev-python/python-exec-10000.2:2::gentoo
Unsuitable candidates:
* dev-lang/python-exec-0.3.1:0::gentoo
Did not meet dev-lang/python-exec:0[python_targets_jython2_5(-)?,python_targets_jython2_7(-)?,python_targets_pypy2_0(-)?,python_targets_python3_2(-)?,python_targets_python3_3(-)?,python_targets_python2_6(-)?,python_targets_python2_7(-)?,-python_single_target_jython2_5(-),-python_single_target_jython2_7(-),-python_single_target_pypy2_0(-),-python_single_target_python3_2(-),-python_single_target_python3_3(-),-python_single_target_python2_6(-),-python_single_target_python2_7(-)], use existing if possible, installing to / from dev-python/python-exec-10000.2:2::gentoo
Flag 'python_targets_pypy2_0' enabled if it is enabled for 'dev-python/python-exec-10000.2:2::gentoo', assuming disabled if missing; Flag 'python_targets_python3_3' enabled if it is enabled for 'dev-python/python-exec-10000.2:2::gentoo', assuming disabled if missing
* dev-lang/python-exec-0.9999:0::gentoo
Masked by keyword
Keywords

Explaining requested decisions:

For dev-lang/python-exec:0::(install_to_slash):
The following constraints were in action:
* dev-lang/python-exec:0[python_targets_jython2_5(-)?,python_targets_jython2_7(-)?,python_targets_pypy2_0(-)?,python_targets_python3_2(-)?,python_targets_python3_3(-)?,python_targets_python2_6(-)?,python_targets_python2_7(-)?,-python_single_target_jython2_5(-),-python_single_target_jython2_7(-),-python_single_target_pypy2_0(-),-python_single_target_python3_2(-),-python_single_target_python3_3(-),-python_single_target_python2_6(-),-python_single_target_python2_7(-)], use existing if possible, installing to /
Because of dev-lang/python-exec:0[python_targets_jython2_5(-)?,python_targets_jython2_7(-)?,python_targets_pypy2_0(-)?,python_targets_python3_2(-)?,python_targets_python3_3(-)?,python_targets_python2_6(-)?,python_targets_python2_7(-)?,-python_single_target_jython2_5(-),-python_single_target_jython2_7(-),-python_single_target_pypy2_0(-),-python_single_target_python3_2(-),-python_single_target_python3_3(-),-python_single_target_python2_6(-),-python_single_target_python2_7(-)] from dev-python/python-exec-10000.1:0::gentoo, key 'Run dependencies', labelled 'RDEPEND'
Because of dev-lang/python-exec:0[python_targets_jython2_5(-)?,python_targets_jython2_7(-)?,python_targets_pypy2_0(-)?,python_targets_python3_2(-)?,python_targets_python3_3(-)?,python_targets_python2_6(-)?,python_targets_python2_7(-)?,-python_single_target_jython2_5(-),-python_single_target_jython2_7(-),-python_single_target_pypy2_0(-),-python_single_target_python3_2(-),-python_single_target_python3_3(-),-python_single_target_python2_6(-),-python_single_target_python2_7(-)] from dev-python/python-exec-10000.2:2::gentoo, key 'Run dependencies', labelled 'RDEPEND'
No decision could be made
For dev-lang/python-exec:2::(install_to_slash):
The following constraints were in action:
* dev-lang/python-exec:2[python_targets_jython2_5(-)?,python_targets_jython2_7(-)?,python_targets_pypy2_0(-)?,python_targets_python3_2(-)?,python_targets_python3_3(-)?,python_targets_python2_6(-)?,python_targets_python2_7(-)?,-python_single_target_jython2_5(-),-python_single_target_jython2_7(-),-python_single_target_pypy2_0(-),-python_single_target_python3_2(-),-python_single_target_python3_3(-),-python_single_target_python2_6(-),-python_single_target_python2_7(-)], use existing if possible, installing to /
Because of dev-lang/python-exec:2[python_targets_jython2_5(-)?,python_targets_jython2_7(-)?,python_targets_pypy2_0(-)?,python_targets_python3_2(-)?,python_targets_python3_3(-)?,python_targets_python2_6(-)?,python_targets_python2_7(-)?,-python_single_target_jython2_5(-),-python_single_target_jython2_7(-),-python_single_target_pypy2_0(-),-python_single_target_python3_2(-),-python_single_target_python3_3(-),-python_single_target_python2_6(-),-python_single_target_python2_7(-)] from dev-python/python-exec-10000.2:2::gentoo, key 'Run dependencies', labelled 'RDEPEND'
The decision made was:
Use origin ID dev-lang/python-exec-2.0:2::gentoo
Install to repository installed

Code:

cave resolve dev-lang/python-exec --explain "dev-python/python-exec"
These are the actions I will take, in order:

Does this mean that I need to add --uninstalls-may-break '*/*' and all the dependent packages will be reinstalled?

There must be something different on your system than on mine. I had a quick look at my bash_history and there is no uninstalls-may-break or something like that anywhere near my command.
From the output I'd guess that uninstalls-may-break will really break these packages, so I cannot advise to do this.